1. Field of the Invention
The present invention relates to a compressor and, more particularly, to a compressor provided with two compression mechanisms of a rotary type compression mechanism and a scroll type compression mechanism.
2. Description of the Related Art
A compressor provided with two compression mechanisms of a rotary type compression mechanism and a scroll type compression mechanism has been proposed. For example, Japanese Patent Laid-Open No. 5-87074 discloses a two-stage compressor in which an electric motor is provided in a single hermetic housing and two compression mechanisms each driven by the rotating shaft of the electric motor are provided; one of these two compression mechanisms is made a rotary type compression mechanism and the other thereof is made a scroll type compression mechanism; and one of the two compression mechanisms is on the low stage side and the other thereof is on the high stage side. Japanese Patent Laid-Open No. 5-87074 describes that in this two-stage compressor, the low stage-side compression mechanism is preferably of a rotary type. According to this two-stage compressor, the low stage-side compressor compresses gases from a low pressure to an intermediate pressure, and the high stage-side compressor compresses gases from the intermediate pressure to a high pressure. Therefore, the drawback of individual compressor is overcome, and a compressor small in size but high in performance can be provided as compared with the case where a rotary type compression mechanism or a scroll type compression mechanism is used singly to compress gases from a lower pressure to a high pressure.
The rotary type compression mechanism has a rotor performing eccentric rotating motion in a cylinder and a blade reciprocating in a groove in the cylinder while the tip end thereof is in contact with the rotor. The blade partitions a space formed by the cylinder and the rotor into a suction chamber and a compression chamber. This blade must be lubricated because of its sliding motion performed when the blade reciprocates in the groove. Therefore, the oil level of lubricating oil is controlled so that the cylinder is immersed in the lubricating oil stored in an oil reservoir provided in the bottom part of the compressor.
From the viewpoint of energy saving, an inverter is used for the rotating speed control of a compressor. The inverter can be operated in a wide range from a low rotational speed to a high rotational speed. In the case of low rotational speed, the quantity of lubricating oil drawn up from the oil reservoir to lubricate the compression mechanisms is small, but in the case of high rotational speed, a large quantity of lubricating oil is drawn up. That is to say, the use of the inverter changes the height of oil level depending on the rotational speed of the compressor.
Also, in recent years, from the viewpoint of the preservation of global environment, the use of carbon dioxide (CO2), which is one of natural refrigerants, as a refrigerant gas has been studied. If CO2 is used as a refrigerant gas, the pressure on the high pressure side of a heat pump cycle increases and exceeds the critical pressure. If CO2 in a supercritical pressure state is used, the dissolution amount of lubricating oil increases, so that the height of oil level is liable to change depending on the operating condition. In particular, in an operating condition in which the circulation amount of refrigerant gas is large, the quantity of lubricating oil in the oil reservoir decreases, and the oil level may become lower than the cylinder. At this time, the lubricating oil is not supplied to between the blade and the groove. Therefore, the mechanical efficiency is decreased by the increase in friction between the blade and the cylinder (groove), and also the reliability may be decreased by the friction. Also, the refrigerant gas flows in between the suction chambers or the compression chambers from the back surface of blade, which also poses a problem of decreased compressing efficiency.